Particle speed and density
Yesterday I found out how to control the particle's speed.
You can do this from "Timestep"! 



You can find it in the "Particles" tab in the "Properties" window. Then go to "Physics" menu. Over there is the "Timestep" slide, which by default is on "0.040" which is quite fast. So if you want slower the particles you can change this to "0.005" or even for very slow motion you can set it to "0.0005".

And most of all!!! You can keyframe the particles "Timestep", so you can do nice and cool animations. I have made a quick demo in which you can see that. It's here: Particles speed test in Blender .

I hope I was helpful!

P.S.
I want to apologize about my bad english (it's not perfect).